Irrigation for Corn and Soybeans

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Warm weather and high heat indexes mean farmers are irrigating fields that have missed rain recently. It is important to consider crop stage and utilize available soil sensor data to make the best decision on when and how much water to apply.  Sensor technology has improved in recent years, and correctly placed sensors prevent over-watering, potentially producing better yields while saving money.  Over-watering also leads to nutrient leaching or loss if soil water level is kept too high for the infiltration potential of the field.  In mid to late June and beyond, water may be needed when available soil water falls below 50 percent in the crop root zone.

Corn benefits greatly from irrigation and pivots should be run early enough in the season to maintain adequate soil moisture.  Later planted corn is between V9 and V13, which is a  critical period of rapid growth and ear size determination. Early planted corn is tasseling, so note silking date for each field.  Peak water demand occurs between tassel emergence and blister stage (about 10 days after silking).  In average rainfall years, irrigate until ears reach the ¾ milk line (about 50 days from silking date).  If July turns hot and dry, we may need to water until black layer (about 65 days from silking date).  Few irrigation systems can meet the maximum water demand for corn, therefore, it is important not to fall too far behind closer to pollination and early grain fill.

Table 1.  Critical Water Demand for Corn: if soil sensors are not used, document rain received and estimate amount of water needed

Growth stage Importance Max crop water use
V9 – V12 Rapid vegetative growth 1.75”/wk
V12 – V17 Ear length determined 2.0”/wk
V17 – R2 (blister) Ear size potential 2.3”/wk
R3 (milk) – R4 (dough) Grain filling period 1.75”/wk
R5(dent) –

R6 (black layer)

Grain weight




* Irrigation may not be needed past ¾ milk line. Under hot, drought conditions producers may benefit from watering until black layer.

For soybeans, irrigate to a stand or apply about 0.5” of water to incorporate residual herbicides in late planted beans.  Soil sensors may benefit soybeans by reducing the likelihood of overwatering a field.  Soybeans should be irrigated for crop stage, but overwatering, regardless of stage, can cause lodging, more disease and lower yield.

Recent work at Milan (good rainfall years of 2014-2015 on a silt loam soil) showed that delaying irrigation on schedule until R3 produced yields similar to initiating irrigation on schedule beginning at R1.  In 2016, Milan was droughty in May and early June, and beans watered on schedule starting at R1 retained more pods per plant at harvest.  However, an increase in lodging (and lower yield) offset any benefit of watering on schedule starting at an earlier growth stage.  Producers who struggle with lodging and lower irrigated bean yields often plant varieties that grow too tall under irrigation or apply too much water after bloom for soil conditions, and soil sensors can do a better job of predicting soil water content (and irrigation need) than the old ‘checkbook method’ of guessing at soil levels based on rainfall.

Terminate irrigation at R6 or full seed stage (50% of plants have touching seed in one of the top 4 nodes).  Adding water beyond R6 may reduce yields based on 3 years of data from Milan.

Table 2. Critical Water Demand for Soybean: if sensors are not used, document rain received and estimate water needed; monitor soil runoff and avoid over-watering:

Growth stage Importance Est. crop water use
V3 – R1 (begin bloom) Rapid vegetative growth 1.0”/wk*
R1 (begin bloom) – R3 (begin pod) Pod count set 1.4”/wk
R4 (full pod) – R6 (full seed) Seed size set 2.0”/wk

* Rainfall is usually adequate until R3 unless fields are under drought

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